Menu Extension for creating Navbars

@phiamo, Instead of creating a builder that extends the AbstractNavbarMenuBuilder, we could do it this way. I think it looks cleaner and it allows you to use the native knp controller style format. Check out the documentation specifically: https://github.com/isometriks/MopaBootstrapBundle/blob/navbar-factory-extension/Resources/doc/4-navbar-generation.md#special-menu-options. 

How concerned are we with being BC? I think this Navbar stuff could definitely be improved a lot but we might some things. This sets the default menu template to be MopaBootstrapBundle:Menu:menu.html.twig so that we can render the icons in the view instead of adding their html in the classes. I think this is better, and it also adds things like carets. Anyway, let me know what you think, I have some more ideas on this.